1. Field of the Disclosure
The present disclosure relates to technology for an inductively coupled plasma mass spectrometer (ICP-MS), which is a mass spectrometer that uses inductively coupled plasma as the ion source, and in particular, relates to technology for analyzing a high-matrix sample.
2. Discussion of the Background Art
The ICP-MS is known as a high-sensitivity analyzer for detecting traces of metal ions. The basic structure comprises a plasma-generating part for generating plasma from a sample such as a liquid, and a mass-analyzing part for extracting ions from the generated plasma and analyzing these ions.
The plasma-generating part, particularly in the case of a liquid sample, comprises a nebulizer for nebulizing a liquid sample of a certain concentration using a gas having a specific flow rate; a spray chamber for isolating some of the nebulized liquid drops in the form of an aerosol together with an appropriate gas; and a plasma torch such that plasma is generated from the plasma gas and the aerosol is introduced into this plasma.
In further detail, the aerosol is generated by at least some carrier gas being introduced into the nebulizer together with the liquid sample. When this portion of carrier gas blows the liquid sample, the liquid sample is nebulized. The nebulized liquid drops circulate inside the spray chamber. The liquid drops that are relatively large in diameter adhere to the inside walls of the spray chamber and are drained, while only the liquid drops that are relatively small in diameter are transferred toward the plasma torch. In essence, the liquid drops of small diameter, together with the carrier gas for nebulization, form the aerosol and are guided to the plasma torch. The carrier gas is usually an inert gas, typically argon gas.
The plasma torch comprises an inside pipe into which the sample that contains aerosol is introduced and one or multiple outside pipes disposed such that they surround the inside pipe. Auxiliary gas, plasma gas for generating plasma, and the like can be introduced into the outside pipes. Once plasma has been generated by the plasma gas through the operation of a work coil, the aerosol comprising the sample is introduced and as a result, the metal in the sample is ionized and blew out into the plasma.
An interface that faces the generated plasma is disposed at the front end of the mass-analyzing part, which is positioned at the last step of the plasma-generating part. The interface has a two-step structure of a sampling cone and a skimmer cone, and each of these has an orifice for extracting the ions from the generated plasma. Extractor electrodes for extracting the ions in the form of ion beam are disposed at the last step of the interface. The extracted ion beam is introduced into the mass analyzer disposed at the last step and the element is identified due to mass/charge ratio. The analysis results can thereby be obtained in the form of a mass spectrum.
Although well-known throughout industry, an example of the overall structure of an ICP-MS is described in the following JP Unexamined Patent Publication (Kokai) 2000-67804, and the structure of the plasma-generating part is described in JP Unexamined Patent Publication (Kokai) 10-188877. Moreover, JP Unexamined Patent Publication (Kokai) 10-208691 describes technology relating to the use of a plasma-generating part and a mass-analyzing part in combination with one another.
A high-matrix sample is an example of a potential sample to be analyzed by such a device. A “high-matrix sample” is a sample that contains the element to be measured as well as water-soluble substances, such as high concentrations of metal salts. Seawater is an example of a high-matrix sample. When a high-matrix sample is analyzed by conventional methods using conventional devices, there are problems in that, as a result of large amounts of ions being guided to the last step of the device, metal salts and the like are deposited and pollute the surfaces of the sampling cone, skimmer cone, etc., and the orifices become clogged, making analysis impossible. It generally is difficult to analyze a sample in normal mode if the matrix concentration or total dissolved solid (TDS) concentration exceeds 1,000 to 2,000 ppm.
On the other hand, by means of an inductively coupled plasma optical emission spectrometer (ICP-OES), it is possible to analyze even a high-matrix sample having a concentration on the percent order or higher without using a diluting means, which is described later. However, ICP-OES have a disadvantage in that their sensitivity or detection limit is inferior, by three decimal places or more, to inductively coupled plasma mass spectrometers, and it is very difficult to satisfy user requirements for quantitative analysis.
JP Unexamined Patent Publication (Kokai) 8-152408 describes a device comprising an optical measuring device and a mass analyzer for analyzing diverse samples having different matrix concentrations. Nevertheless, the structure of the device described in JP Unexamined Patent Publication (Kokai) 8-152408 is impractical because it is complex and not easy to hold or manipulate, and it does not solve the problem of quantitative analysis.
A single ICP-MS capable of high-sensitivity analysis of liquid samples having a wide range of matrix concentrations would be very effective for practical use. The method whereby a highly concentrated sample that cannot be analyzed directly is diluted to an acceptable extent before aerosol generation is one example. Dilution can be carried out manually or automatically using an autodiluter. JP Unexamined Patent Publication (Kokai) 11-6788 gives an example of a method for diluting a liquid sample using an autodiluter.
Nevertheless, performing dilution by hand takes time. Diluting many samples is an inconvenience in terms of time, and there is also the chance that there will be errors in dilution. On the other hand, using an autodiluter complicates dilution by adding a step for operating additional equipment, and there is the chance that the sample will be contaminated by the outside environment or the tools that are used during dilution of the liquid sample.
An object of the present disclosure is to provide an inductively coupled plasma mass spectrometer with which a user can analyze, continuously and with good reproducibility, samples of various concentrations, including high-matrix samples, without implementing a manual procedure.